The present invention arose from experiences encountered in the pressure-testing of certain catalyst-loaded reactors with nitrogen gas. In the commercial startup of processes utilizing high-pressure hydrogen, e.g. hydrocracking, it is common practice to carry out a preliminary pressure test of the high-pressure system, including a catalyst-loaded reactor, in order to insure against possible leakage of combustible gases during subsequent processing. Commonly, for safety reasons an inert gas is employed for this pressure testing, nitrogen being preferred for economy reasons. For most operations nitrogen performs very satisfactorily, but for reasons which are still largely conjectural, at elevated temperatures (above 200.degree. F) nitrogen, in the absence of oxygen, has been found to have a substantial adverse effect upon certain catalysts comprising a Group VIII noble metal supported on crystalline aluminosilicate zeolites, wherein the zeolite support has been substantially converted to a hydrogen and/or dehydroxylated form. In some cases, it has been found that after undergoing nitrogen pressure testing, the activity of such catalysts for hydrocracking and/or hydrogenation is drastically reduced, to levels of only a small fraction of the initial fresh activity.
Surprisingly, seemingly analogous catalysts based on metal-stabilized crystalline zeolites, such as magnesium-stabilized zeolites, do not appear in their fresh state to be affected adversely by high-temperature nitrogen. Another puzzling aspect of the invention is that the damage appears to occur only when the catalyst is in an oxidized state (as after oxidative regeneration or air calcination of the fresh catalyst); in its reduced state, little or no damage occurs.
In view of the foregoing, it would appear that the above noted damage could be avoided by either of two possible expedients. Firstly, since the nitrogen damage becomes significant only at temperatures above about 200.degree. F, pressure testing could be carried out at temperatures sufficiently low to avoid significant damage. However, this alternative would be hazardous in many cases due to the "hydrogen-embrittlement" which many catalytic reactor composed of ferrous alloys may have previously undergone through extended use at high hydrogen pressures. When such embrittlement occurs, the "transition temperature" of the reactor walls may substantially increase, to levels in the range of about 200.degree.-300.degree. F. The transition temperature is the temperature below which cracks in the reactor wall will propagate (at a given operating pressure) in an instantaneous, catastrophic manner, and above which cracks will be arrested by the inherent toughness of the metal. It is hence considered a prudent safety measure to conduct pressure testing (at pressures in excess of about 700 psig) only at temperatures above about 200.degree. F, and preferably above about 300.degree. F.
Secondly, as disclosed in U.S. Pat. No. 4,064,036 to nitrogen damage can be avoided by pre-reducing the catalyst at low temperatures and pressures with a dilute non-combustible mixture of hydrogen and nitrogen prior to the pressure testing. This procedure permits prereduction to be carried out safely without danger of fires or explosions in the event of leakage from the reactor, and the resulting catalyst is not damaged upon subsequent pressure testing with nitrogen. However, it is not always convenient to carry out such a prereduction; and in other cases a need may suddenly arise to blanket the hot catalyst with nitrogen to isolate it from some reactive gas such as hydrogen, oxygen, or hydrocarbons. In such cases, a post-treatment would be desirable to reverse any damage which occurs.
I have now discovered that nitrogen damage of the type noted above can be substantially completely reversed by subjecting the catalyst to a simple, relatively short oxidation treatment with an oxygen-containing gas at elevated temperatures and relatively low pressures. The efficacy of this treatment is very surprising in view of the fact that the catalyst, prior to contact with nitrogen, was already in an oxidized state which presumably would not be altered by contact with nitrogen. Apparently however, this particular type of catalyst does catalyze the production of some type of oxidizable component upon contact with nitrogen at elevated temperature, a phenomenon which does not occur with other seemingly analogous catalysts.